Plasma and Thermal Processing Leading to Spatial and Temporal Variability of the Trapped O2 at Europa and Ganymede
We describe the physical processes that affect the formation, trapping, and outgassing of O2 at Europa and Ganymede. Following Voyager measurements of their ambient magnetospheric plasmas, laboratory data indicated that observed ions, mostly ejected from volcanic Io, would in turn impact and sputtering their surfaces, decomposing the ice producing thin oxygen atmospheres. Subsequently, Europa and Ganymede's O2 atmospheres were inferred from O aurora, condensed O2 bands identified at 5773 and 6225 Angstroms, and their atmospheres were shown to have a dusk/dawn enhancement, confirmed by recent Juno data. Although plasma produces these observables, processes that occur within the topmost surface are not well understood. Here, we note that the incident plasma particles produce nonequilibrium defect density locally in the surface ice grains. Defect diffusion within these grains leads to the formation of voids and molecular products, some of which are volatile. Although some volatiles are released into the satellite atmospheres, others are trapped at defect sites or trapped in voids, creating bubbles whose lifetimes are limited by the plasma-induced destruction rate. We discuss how trapping competes with annealing of the radiation damage, and how hemispheric differences at Europa and Ganymede, roughly determine the observed trend with latitude of O2 bands. We discuss the relative importance of condensed O2 and O2 adsorbed on regolith grains as atmospheric sources, accounting for dusk/dawn enhancements and temporal variability reported in condensed O2 band depths. Since plasma-induced damage and thermal annealing timescales drive oxidant variability on icy moons (likely also Callisto, Dione, and Rhea), they can help determine volatile downwelling, a potentially metabolic source for their oceans, and upwelling of other trapped oxidants (e.g. CO2) suggestive of ongoing geologic activity.